This invention relates to resistive ladder networks; and more particularly to circuits for compensating for inaccuracies caused by resistive variations within the ladder. These resistance variations are unintentional, and are due to various processing limitations. In an ideal ladder network having no resistance variations, current is precisely divided in two as it passes from stage to stage. However, in an actual ladder network, resistance variations within the stages cause this current division to be upset somewhat.
In the past, various circuits and methods have been devised to "trim" the actual resistors to their nominal value. One such prior art method is known as laser trimming. Basically, with this method, portions of the thin film that comprises the resistor are selectively removed by a laser beam. In another prior art method, small resistive regions are selectively added in series to a large resistive region that is to be trimmed. Both of these methods, however, are unsatisfactory for various reasons which are explained in greater detail in the Detailed Description portion of this disclosure.
Therefore, it is a primary object of the invention to provide a resistive ladder network with an improved resistance trimming circuit.